This invention relates to rotary vacuum filters and more particularly to a system and method for more effectively causing a pressurization or blow to discharge a cake of solid particulates accumulated during a continuous filtering process. Rotary vacuum filters of the disc-type or drum-type are useful in a wide variety of applications, especially in continuous filtering situations where large quantities of filtrates and particulate materials must be separated. Examples of such uses are in sugar processing and paper making. An application which is of considerable importance is the removal of coal from a coal-water slurry after the coal has been transported over extended distances through a pipeline or in coal preparation plants.
While certain of the inventive concepts are applicable to both disc-type and drum-type rotary vacuum filter systems, the inventive concepts are illustratively disclosed herein in a disc-type system. Rotary vacuum filters typically involve a number of filter segments arranged to extend from a central shaft or core member. In disc-type rotary vacuum filters, a number of axially spaced disc means are provided, and each disc means is formed by a number of filter segments extending radially outward from the core somewhat like sectors. Communication channels are established through the core which connect to certain groups of filter segments in the disc or drum structure. The core member is adapted to be rotably supported in a horizontal position slightly above a bath of liquids and particulate solids or a slurry. Rotation of the core with its attached filter segment structure causes each of the filter segments to be successively submerged in a slurry for somewhat less than one-half of a revolution.
As the filter segments are rotated into the slurry, the communication passage connecting those filter segments is opened to a source of vacuum. The vacuum attracts the particulates to the outside filter media of each filter segment while the filtrate moves through the filter media and into the filter segments and communication channels. The particulates of the slurry held on the filter media of the filter segment by the vacuum results in an accumulation of a cake or layer of particulates. The filtrate is removed from the communication channels and collected. In this manner, the filtrate is separated from the solid particulates, and either the separated filtrate or the solid particulates may be used in a desired situation.
The cake adheres to the filter segments after they rotate out of the slurry and after the vacuum has been removed because of surface tension forces of the cake interlocking with the filter media. This cake then must be removed before the filter segments again introduced into the slurry during the succeeding revolution by provision of removal means such as a scraper mounted stationarily and closely adjacent to contact the cake on the filter segment structure at a desired point during rotation. To further facilitate removal and discharge, a source of compressed air is connected to the communication passage to pressurize or blow each filter segment which is about to or is undergoing scraping. The blow returns a small amount of residual filtrate to the filter media to wet it which causes a loosening or breaking of the surface tension forces that cause the cake to adhere to the filter media of each filter segment. Prior art blow systems have been relatively slow in operation and have not adequately loosened the cake, particularly when the filter media of the filter segment is very porous or open and when the residual materials of the cake are wet and pliable.
The slowness of prior art blow systems is a result of the manner of application of the compressed air to the filter segments. In the usual prior art arrangement, one or both ends of the core of the rotary vacuum filter are terminated with valve ports opening to the communication passages to the filter segments. A stationary valve bridge plate is located adjacent the valve ports of the rotating core, and vacuum and blow parts are located at selected positions in the stationary valve bridge plate to provide vacuum or pressure to the valve ports at selected positions of rotation of the core. As the core rotates, the prior art valve ports of the core are slowly and gradually exposed or opened to the stationary vacuum and blow parts of the bridge plate thereby causing gradual application of pressure or vacuum at those filter segments whose communication passages are connected through the gradually opening valve ports. The slow and gradual opening of of the valve ports between the core and the stationary bridge plate results because of the slow rotation of the core, since it may take up to ten seconds for a port to open fully. During the period of this slow opening, there is a considerable energy loss due to a velocity head loss as the compressed air flows through the gradually opening valve ports to effect the flow. This velocity head loss requires that the air pressume source or pump provide higher than necessary pressure so that sufficient energy to blow the filter segments remains after the velocity head loss dissipation.
Increased pressure and volume necessary to overcome the velocity head loss has required the use of larger and more powerful motors for the pressure pumps to provide the increased energy. The motor driving the pressure pump operates continuously to supply air to the filter and, when a maximum pressure is reached prior to the blow, a relief valve connected to the pump opens to prevent any further pressure increase. This causes an oscillatory operation of the motor under cyclically fluctuating and recurring heavy loads to build up the pressure between successive blows. This cyclic operation has required relatively large motors which can withstand such operation without overheating and destruction.
A disadvantage of the slow blow type of prior art system utilizing a lobe type blower has been that it is difficult to make such a system conform to the noise standards as are currently applicable for the safety of workers under the Occupational Safety and Health Act. The cyclic operation of the motor and the operation of the air pressure pump driven by the motor may sometimes occur at natural harmonic frequencies of the structure surrounding the rotary vacuum filter causing a dramatic amplification of the noise levels at that harmonic frequency, particularly when lobed or Roots-type pressure pumps are employed. In addition, the pressure relief valve repeatedly opening to allow the escape of air causes surges of air with attendant increases in the noise level.
Prior art suggestions and attempts to solve the noise problem with slow blow systems, have included the substitution of slower and quieter running centrifugal blowers for the usual lobed air pressure pumps. Such substitution is generally unsatisfactory. The cyclic accumulation of pressure and its release by the relief valve causes centrifugal blowers to surge, and surging causes accelerated wearing of the mechanical parts and consequent reduced lifetime and increases susceptibility to failure. When slower running lobe blowers are employed, larger and more costly motors must be used. Furthermore, such substitution does not eliminate the noise from the pressure relief valve.
Another significant disadvantage of slow blow systems is that they are not always effective in loosening or dislodging the cake over the total surface of the filter media of each filter segment. The slow opening of the valve ports cause the filter segments to be pressurized slowly, and this slow application of pressure tends to remove or loosen the cake on the filter media of the filter segments only at small portions of the filter media. After this partial loosening or dislodgment occurs, a largely unrestricted opening through the filter media to the atmosphere is opened for the increasing air stream of the pressurized air, which conducts the air stream from the filter segment. Thus, the intended total loosening that would occur if the filter segment could be pressurized more quickly does not occur. It is desirable to loosen or dislodge the whole cake approximately simultaneously before it encounters the scraper to facilitate a better removal, but due to the slow opening of the valve ports in slow blow systems, the desired performance is not achieved.
Snap blow systems, also known in the art, involve very high pressure and low volume sources of air, and are used with filters having very fine filter media such as cloth bags. Snap blow systems involve expensive high pressure pumps, accumulator tanks and pressure relief valves and employ an arrangement of control valves to delay the application of the pressure to the filter segments until the valve ports have achieved a full opening. However, snap blow systems are not of a nature that can be used with porous or coarse filter media such as wire mesh because the filter media must be very fine to provide the requisite resistance to the high pressure so that the whole filter segment can be pressurized very quickly. The fine filter media only allows snap blow systems to be used in certain applications, for example, to remove copper or iron, and the applications are generally effective only when the cake discharge is dry and brittle. If the cake discharge is somewhat moist and pliable, as in the case of coal taken from a slurry, snap blow is not greatly effective. With a fine filter media, it may be difficult to obtain a high volume of cake discharge as in the case of removing coal from a slurry at a coal preparation plant where, for example, 40 to 50 pounds of coal must be removed each hour from each square foot of filter media of the filter segments.
A problem which may be encountered with low pressure, high-volume air blow systems is an undesirable amount of blow-back of residual filtrate during the blow. Although some blow-back of residual filtrate particles is effective in loosening the cake through reduction of surface tension of the interlocking surface of the cake with the filter media, too much blow back so sufficiently wets the cake that the separated particulates of the cake contain an undesireably high content of filtrate. This, of course, reduces the effectiveness of the separation process and is to be avoided.
Accordingly, it is a general object of this invention to provide a blow system and method for rotary vacuum filters which avoid the foregoing disadvantages of the prior art and allow the effective use of rotary vacuum filters in applications which have heretofore not proved largely successful.
It is an object of this invention to provide a bypass and synchronized blow system and method which avoid energy dissipation inherent in the velocity head losses resulting from the slow opening of the valve ports.
It is another object of this invention to provide a bypass and synchronized blow system and method which quickly and effectively pressurize the filter segments.
It is another object of this invention to provide a bypass and synchronized blow system and method which effectively distribute the air pressure throughout the whole surface of relatively porous filter media of the filter segments.
It is still another object of this invention to provide a bypass and synchronized blow system and method which cause a uniform and mearly simultaneous loosening or discharge of the accumulated cake.
Another object of the invention is to provide a bypass and synchronized blow system and method which more effectively loosen and dislodge the whole cake accumulated, to thereby secure a more effective performance of low-pressure high-volume blow systems for rotary vacuum filters.
This invention of a bypass and synchronized blow system and method involves another of its objectives the reduction of noise during the blow of rotary vacuum filters.
Another objective of this present invention is to provide means associated with a rotary vacuum filter for eliminating the undesirable effects associated with excessive blow-back of filtrate during the blow.
To achieve these and other advantages and objectives, the present invention involves an airstream control means, which may include a bypass apparatus, in conjunction with a synchronizing apparatus for connecting blower air to the filter segments only when the valve ports through which the airstream must pass have rotated into a relatively unrestricted opening with a blow port in a stationary bridge plate. A bypass means is provided for directly connecting the pressurized air obtained from a continuously operable pump or blower to the atmosphere when a blow is not required. The synchronizing apparatus is operative in conjunction with the bypass means to conduct the airstream through the ports only when there is a relatively unrestricted opening therethrough, at which time a bypass valve is operable to simultaneously and rapidly terminate the discharge of the airstream from the pump to the atmosphere and, instead, direct it through the unrestricted opening of the ports into the communication channels to the filter segments, thus avoiding velocity head losses. A trap or pocket is provided in the communication passages to eliminate undesirable amounts of filtrate which could be blown back during the blow.
A more complete understanding of the present invention may be had by a reference to the following detailed description of the invention and the brief description of the drawings in which: